1. Field of Use
This invention relates generally to control means for large cranes, such as tower cranes which employ a swingable, luffable crane boom and a boom support tower.
In particular, it relates to a control means which employs load sensing devices to sense load conditions at various locations in the crane, which further employs a programmable electronic computer which calculates load moments at such locations, and which also provides output signals related to the load moments, which signals are usable to operate the crane within safe operating limits.
2. Description of the Prior Art
A typical mobile tower crane generally comprises a self-propelled vehicle, a boom support tower extending vertically or near vertically upwardly from the vehicle and mounted thereon for rotation about a vertical axis, and a crane boom extending horizontally outwardly from the boom support tower and mounted thereon for pivoting (luffing) about a horizontal axis. In operation, swinging of the crane boom is effected by rotating the tower left or right and luffing of the crane boom is effected by pivoting the crane boom up or down on the tower. In operation, a load to be lifted, swung and lowered is attached to a load line suspended from a pulley on the point end of the boom. The tower and boom are each on the order of 50 feet or more in length. Therefore, for weight reduction purposes and to facilitate set-up and disassembly of the crane on a job-site, the tower and boom each comprise a plurality of lattice-type sections which are mechanically joined end-to-end by removable pins or bolts. Such a crane, because of its large size and configuration, can be subjected to severe load conditions which can cause internal structural damage, such as bending or collapse of longitudinal chord members or cross braces of which the lattice-type sections are constructed, or even cause the crane to tip over. These severe load conditions can occur either while the crane is in operation or while the crane is stationary and not in operation.
In the following discussion, it should be understood that the load condition which is of concern is not merely the magnitude of a force applied at a certain point on the crane structure, but the load moment, i.e., the magnitude of the force times the distance between the point at which the force is applied and some known fulcrum point. The force can be a function of the weight or mass of the crane components, the weight or mass of the load being handled by the crane, the wind or other force acting on a side of the boom and the load, or any combination thereof.
Some of the most common operating practices or conditions which are likely to cause damage are: attempting to lift too heavy a load, operating a crane which is not properly leveled, attempting to lift a load while the boom is at an improper luffing angle with respect to the size of the load (i.e., too low); accelerating or decelerating too rapidly while raising or lowering a load or while hoisting or lowering the boom or while swinging the boom; unintentionally forcing the boom (vertically or horizontally) against some fixed object or structure, and securing the boom in such a position that side loads are imposed thereon by prevailing winds. Side loads are also induced in the boom due to deflection of the crane under structure and the deflection of the boom. Unacceptable forces can be applied to the boom alone and/or to the tower.
Crane manufacturers endeavor to take these factors into account when designing and building cranes so that a given crane will be in conformity with industry standards and OSHA regulations. Nevertheless, due to the complexity of the total crane structure, it is still possible to operate the crane beyond safe limits, operating instructions specifying certain not-to-exceed limits are provided for the operator and, in some cases, control systems are embodied in the crane either to warn the operator when he is about to exceed safe limits or to initiate certain control functions which automatically prevent safe limits from being exceeded Furthermore, some crane operators develop operating practices, based on experience, to aid them in staying within safe limits.
During crane operation, the boom is often subjected to side loads, as hereinafter explained. In a tower crane wherein tower rotation effects boom swing, the tower itself is subjected to torque loads generated by boom side loads. Side loads of sufficient magnitude can damage or collapse the boom or tower or both, especially in lattice-type cranes, or even overturn the crane. Side loads result from various causes, such as misalignment of the suspended load relative to the boom point end, or accidentally swinging the boom directly against some nearby object or structure location of the crane on uneven terrain, or wind.
As an example of load misalignment, when the boom is swung in a given direction (left or right) with a heavy load suspended therefrom, the initial horizontal motion of the boom "leads" the initial horizontal motion of the suspended load in the same direction. The load tends to "lag" because of inertia. This results in vertical misalignment between the boom point and the load hoist cable attachment point on the suspended load, i.e., the suspended load is initially out-of-plumb on one side of the boom point. Such misalignment imposes side loads on the lateral sides of the boom. More specifically, the structural members (such as chord members) defining the leading side of the boom (i.e., that side farthest from the misaligned load attachment point of the cable) are subjected to longitudinal tension forces. At the same time, the structural members (chord members) defining the lagging side of the boom (i.e., that side closest to the misaligned load attachment point of the cable) are subjected to longitudinal compression forces. When boom swing motion in the aforesaid given direction decelerates and/or stops, the horizontal motion of the suspended load continues in the aforesaid given direction. As a result, the suspended load then becomes out-of-plumb on the opposite side of the boom (i.e., the aforesaid leading side), and the side load on the boom becomes reversed, i.e., compression forces occur on the formerly leading side of the boom and tension forces occur on the formerly lagging side of the boom. As the suspended load swings or oscillates (with decreasing amplitude) relative to a motionless boom point, the side load shifts back and forth between the boom sides (in decreasing magnitude) until oscillation stops and the side load ceases to exist.
Side loads are of particular concern in large multi-section lattice-type booms wherein each lateral side is defined by elongated, usually tubular or angled steel, upper and lower chord members which are joined together at intervals therealong by tubular cross braces or lacing. In tower cranes where the boom is attached to the upper end of and rotatable with a large multi-section lattice-type tower, a side load on the boom generates corresponding torque in the tower, which exhibits itself as twisting of the vertically disposed longitudinal tubular chord members of the tower and compression and tension forces in the cross-braces interconnected between the chord members.
Generally speaking, and assuming a suspended load of given weight suspended by a load hoist cable from the boom point end, side load magnitude increases as a function of any or all of the following factors: an increase in boom length, an increase in boom elevation angle, an increase in the vertical distance between the boom point end and the suspended load attachment point (load hoist cable length), an increase in the rate of acceleration or deceleration of boom swing motion, wind loads, out-of level conditions, or any combination of these factors. Federal OSHA regulations and industry standards specify the type and magnitude of various loads a crane must be able to withstand. In addition, operating data based on calculations and field tests of specific cranes are furnished to the crane operator and define permissible limits of various boom positions and operations (and combinations thereof) which would affect side loading, such as boom hoist angle limits, boom swing position limits, swing acceleration and deceleration rates, load weight limits, levelness and so forth. However, even with such data, practical experience in crane operation is essential to avoid exceeding permissible limits. Frequently, the crane operator develops operating practices based on his experience and knowhow to aid him in staying within limits. For example, one useful practice developed and employed by some operators of small and medium sized cranes is for the crane operator or his assistant to observe or "sight" the angular position of the load line relative to the vertical axis of the crane boom and to control boom swing motion (acceleration and deceleration) so that the load line never goes out-of-plumb for a distance greater than the width of the widest portion of the boom. However, this practice of "sighting" is safe only if the crane is level and is entirely unsuited for large cranes, especially tower cranes, wherein a long boom (on the order of 50 or more feet in length) is mounted on the top of a high vertical tower or mast (on the order of 50 or more feet in height) and visual cues are difficult or impossible to obtain.
Therefore, there is need for a means or system for sensing actual side load conditions and other relevant conditions and for providing this information in a form which can be used by the crane operator to operate the crane within acceptable safety limits.
The prior art discloses various types of load sensing systems and devices for use on cranes. However, insofar as applicant is aware, no system is known, disclosed or available for sensing crane boom side loads and for providing data relative thereto, alone or in combination with other relevant data, such as boom angle and load weight, on which crane operation can be based. The prior art also does not disclose side load sensing systems which consider torque loads imposed on crane towers as a result of boom side loads. The following patents generally illustrate the state of art of sensing systems used in cranes and other machines to sense various loads, load moments and stresses: U.S. Pat. Nos. 3,505,514; 3,638,211; 3,695,096; 3,740,534; 4,535,899 and 4,532,595.
This prior art discloses various types of sensors or transducers to sense conditions at various locations (i.e., boom angle, load) and discloses various types of electronic computers to calculate loads and/or load moments and provide output signals which are usable alone or in conjunction with other data. Some of the prior art patents use various types of visual displays to guide the operator U.S. Pat. Nos. 4,532,595 and 3,638,211 employ a strain gauge type sensor in cables which are employed in the crane to sense loads at certain points. U.S. Pat. No. 3,695,096, like U.S. Pat No. 3,638,211, discloses a strain gauge embodied in a bolt or pin used to secure two mechanical components together. The prior art patents are primarily concerned with measuring loads or load moments acting on the boom in a vertical plane, with the boom at some known luffing angle, although some other conditions are sensed as well.